Sheared gold ore from the David Bell Mine,

Hemlo gold deposit, Ontario

"Rock of the Month # 95, posted May 2009" ---

A sheared gold ore rich in distinctive green mica,
from the large Hemlo gold deposit, located north of Lake Superior,
beside the Trans-Canada Highway (Hwy. 17) in northwest Ontario.
Sample 1607, a biotite- green mica schist, from
the eastern part of the Hemlo orebody, was found in
the David Bell mine (5 level, H3 stope, collected
in situ, 23 March 1994). A spectacular biotite
schist with abundant green mica, with well-developed
crenulation cleavage. Note the "foliation fish"
composed of brilliantly colourful green mica. The rock
appears to be composed largely of quartz and
feldspars, biotite, the green mica, pyrite, and
possibly unseen barite, given a significant "heft" to
the sample.
Tom Muir (Ontario Geological Survey, pers.commun., 2009)
notes that this sample displays exceptionally fine
"green muscovite foliation fish... Given the appearance of the specimen
and the presence of a crenulation cleavage, it strongly suggests that it is
related to the `D3' dextral shear event (following the)
`D2' sinistral event" (see Muir, 2002, 2003 ; also Lin, 2001 and Davis and Lin, 2003).

Indicators for mineralization at Hemlo include green
micas, molybdenite, barite and realgar (see, e.g.,
Harris, 1989). Pyrite is the predominant sulphide. A
pyritic ore from the Williams mine, on the west end of
the orebody, was featured as
Rock of the Month 80.
The references that
follow include a partial update of those quoted in
R.O.M. 80.

Hemlo ore-zone green micas have been age dated at 2671
Ma, probably the event in which isotopic equilibrium
was established between S in pyrite and barite, and O
in quartz and muscovite, at about 500°C (Crocket
et al., 1994).
Green mica rich in V characterizes ore-grade mica schists. The V content of
Hemlo muscovites varies from electron microprobe detection limits up to at
least 17.6 wt.% V2O3 in
roscoelite (Pan and Fleet, 1992a,b; Fleet and Pan, 1998).
The principal substitution is evidently of trivalent
vanadium for aluminium. The host schist was probably
derived mainly from quartz-oligoclase porphyry. The
high V and Cr contents probably stem from alteration
of mafic-ultramafic rocks, and these metals became
fixed in green mica and oxides during waning regional
metamorphism. Later hydrothermal alteration then
mobilized V anew, and generated the V-rich
calc-silicates.

Schnieders and Smyk (1994) reviewed the history and
development of the Hemlo mining camp, more or less at
the peak of the mines' productivity, with annual
production and tonnes/day milled for the 3 mines, from
west to east, standing at ~450,000 oz / 6,000
T/day [Williams], 440,000 oz / 3,000 T/day [Golden
Giant] and 200,000 oz / 1,000 T/day [David Bell]. The
mines supported 1,200 direct jobs, and many more
indirectly across the region. Today, although the
Golden Giant has closed, the good news from the
Williams mine lies with new resources delineated,
which should expand the mine life at least until 2013.
The earliest known exploration in the area involved
Moses Peekongay and Donald McKellar in the 1860s and
1870s (Schnieders and Smyk, 1994). In the early 1930s
J.E. Thomson recognized the potential of the Hemlo
area. Trevor Page explored the area in late 1940s. Ken
Fenwick of the Ontario Department of Mines suggested
remapping in 1975.
However, mining success at Hemlo was hard to achieve,
underscoring the difficulties of exploration. From an
early gold discovery in 1944 until the main ore zone
was found in 1981, over 20 companies and individuals
explored the area, in work which included 9 drill
programs and over 130 drill holes. The pivotal
discovery in May 91 came in hole 76 of the drill
program, a case study of perseverance.

SOME RECENT REFERENCES (2004-2008) AND
CITED ARTICLES ON THE HEMLO GOLD DEPOSIT

Heiligmann,M, Williams-Jones,AE and Clark,JR (2008)
The role of sulfate-sulfide-oxide-silicate equilibria
in the metamorphism of hydrothermal alteration at the
Hemlo gold deposit, Ontario. Econ.Geol. 103, 335-351.